Photo-radio system



J. N. WHITAKER PzIoToRADIo SYSTEM Filed Feb. 26, 1938 lmnnAAA April 8,1941.

mGwSQm. NQS@ S u INVENTOR. J. NA WH/TAKER 7x65 A TTORNEY.

Patented Apr. 8, 1941 PHOTQ-RADIO SYSTEM James N. Whitaker, Tuckahoe, N.Y., assigner to Radio Corporation of America, a corporation of DelawareApplication February 26, 1938, Serial No. 192,732

.(Cl. FX8-5) 6 Claims.

The present invention relates to a method of and means for transmittingintelligence, which is preferably in the form of a picture record, to aremote point where an electro-optical image or facsimile of thetransmitted subject is to be reproduced. Generally, the invention is onewhich is capable of transmitting any type of intelligence, but itisparticularly adaptable to the transmission of subjects which can beobserved by the eye. These subjects, for example, may be pictures,copies of newspapers, financial statements, lm records such as motionpicture film with or without sound equipment, fashion plates, copies offingerprints, drawings or other similar matter.

Briefly, the invention will herein` be characterized as a constantfrequency variable dot system by the use of which printed matter or likematter may be transformed into a series of electrical impulses whichwhen received over a radio or wire line communication channel may beutilized for reproduction purposes. This constant frequency variable dotsystem of transmission consists essentially in the conversion ofamplitude changes of a carrier or tone frequency into varying weightsignalling or marking impulses which are transmitted at a constantfrequency and amplitude so that the useful period of transmission whichwill at the receiving point correspond to marking periods, may have avalue varying between Zero and 100%.

In previously used systems for accomplishing the transmission desired,the lighter portions of the subject to be transmitted, that is, theportions of the subject varying in intensity between middle gray andwhite, for example, the marking periods are generally of constantduration while the spacing periods vary, and as the intensity of thelight or shadow on the subject varies between middle gray and black, foreX- ample, the type of marking to reproduce these particular tonalvalues of the subject changes to a marking of variable duration andequal spacing between each marking representation. With this type ofmarking, to denote the lighter portions of lthe picture between middlegray and white, the relationship between the marking and the spacingperiods may vary .from equality of marking and spacing periods to asmuch as fifty times the time duration of spacing to the time duration ofmarking. Likewise for the darker portions between middle gray and black,the time duration of the marking impulses with respect to the spacingimpulses may vary from equality to fifty times the time duration of thespacing impulses. Thus it will be seen that in addition to varying theweights of the marking impulses the frequency at which these impulsesare transmitted is varying, and with this type of equipment fortransmitting records the variation in frequency range may be, forexample, between and 350 cycles per second.

In another type system which is known, the marking impulses are alwaysat a constant frequency so that the number of marking impulses per unittime is constant, that is, the sum of the marking and spacing durationfor any value of density is always constant but the marking durationdepends upon the intensity of the subject at the point of scanning.Suppose, for example, that it is desired to transmit dots per second andfor a light portion of the subject the marking interval might be 1/10 ofthis period or M500 of a second, then it is seen that the spacingduration would be V150() of a second. However, for a dark portion of thesubject, the conditions might be` such that the marking pulses occur for7/1500 of a second, whereas the spacing would be reduced to 371500 of asecond. In both of these cases it would be noted, however, that the sumof the marking and' spacing time periods is always lm of a second, or ifthe 150 dots per second first assumed is thescreen frequency, then it isseen that the sum of the markingand spacing impulses per time unit isalways the reciprocal of the screen frequency.

In the use of this system in known apparatuses, a screen tone converterand amplifierhas been used to convert the output from the scanning headinto impulses of screen tone frequency andvof varying duration inaccordance with the light and dark values of the subject being scanned.In such systems some difficulty has been encountered inasmuch as theregenerative square wave amplifier will oscillate when the amplitude ofthe modulated scanner carrier plus the applied screen frequency is notsufficient to produce ample voltage changes in the control electrodecircuit of the square wave amplifier. A system such as the one justdescribed is shown and described in detail in the patent to I-IenryShore and James N. Whitaker #2,083,245 of June 8, 1937.

The present invention relates to a material improvement over the systemshown and described in the above mentioned patent in that the system hasbeen substantially simplified and in that the tendency for the system tooscillate has been completely obviated by the omission of theregenerative square wave amplifier.

In the present system, signals of scanner carrier frequency which aremodulated in accordance with the light values of the subject beingscanned, are supplied to an amplifier tube which in turn supplies powerto a rectifier, the output of which represents, in current and voltage,the light and dark values of the particular subject. A screen frequencyof a constant value is then supplied to a mixing tube together with theoutput of the rectifier tube which results in the production of impulsesof constant frequency but of variable intensity, the intensity beingdetermined by the output of the rectifier tube in accordance with thelight and dark values of the subject being scanned.

The output from the mixer tube is then supplied to a push-pull keyerthrough an appropriate phase reversal system in order to accomplish truepush-pull operation of the keyer. The separate elements of the push-pullkeyer are made conducting or non-conducting depending upon the length oramplitude of the output from the mixer tube, which impulses, as statedbefore, are of a constant frequency. The output of the push-pull keyerwhich comprises a series of impulses of constant frequency and constantamplitude but of varying lengths, depending upon the light and darkvalues of the subject, is then supplied to a modulator for modulatingthe line carrier frequency prior to transmission by radio or linechannel. k

It is therefore one purpose of the present invention to provide meanswheieby pictures or printed mattei' may be transmitted by transformingthe light values of the subject into impulses of constant frequency butof varying duration.

Another purpose of the present invention resides in the provision ofmeans whereby a simple and improved method of photo-radio transmissionmay be accomplished.

A still further purpose of the present invention resides in meanswhereby the subject matter to be transmitted by facsimile may betransmitted either negatively or positively.

A still further purpose of the present invention resides in the use of apush-pull keyer for operating a push-pull modulator in a photo-radiotransmitting system.

Another purpose of the present invention resides in the provision ofmeans whereby positive or negative transmission may be accomplished.

Still another purpose of the present invention resides in provision ofmeans for selecting either positive or negative transmission, the meanscomprising a single-pole double-throw switch.

A still further purpose of the present invention resides in theprovision of a circuit whereby negative or positive transmission may beselected by a switching operation without the necessity of changing anyof the adjustments or parameters of the circuit.

Still further purposes and advantages of the present invention willbecome apparent to those skilled in the art from a reading of thefollowing specification and claims, particularly in connection with theaccompanying drawings, wherein like reference characters represent likeparts and wherein:

Figure 1 is a diagrammatical showing to represent the sequence of thevarious operations of the present invention, and

Figure 2 shows a preferred system which is constructed in accordancewith the present invention.

Referring now to the drawings, in Figure 1 is shown a subject l0, whichvaiies in shading from` light to dark or black. It is upon this subjectthat light is projected for reflection on to a light responsive elementfor producing current and voltage variations in accordance with thelight values of the subject. The light responsive element is containedwithin the scanner Il which is supplied with a scanner carrier frequencyby the element l2, the frequency being controlled by a standard constantfrequency. The output from. the scanner is then directed to an amplifieri4 and the wave form of the energy supplied to the amplifier is such asthat shown at i5 for a subject having the light values such as thatshown at I0. It will be noticed by referring to the wave form l5 thatthe wave consists of an amplitude modulated wave, the carrier frequencyof which corresponds to the scanner carrier frequency and the amplitudeof which corresponds to the amount of light which is reflected from thesubject. After the signals have been amplified, they are supplied to arectifying device I6 where they are rectified, the output of this devicebeing represented by the curve IT. It will be noticed by examining thewave form of the curve shown at l1 that it conforms to the envelope ofthe modulated scanner carrier frequency Wave shown at l5.

The rectified signal is then directed to a mixing and amplifying deviceI8 upon which is also impressed a screen frequency of a wave form suchas is shown at I9, and which is produced by a screen frequencyoscillator 20, the oscillator being controlled by a constant frequencystandard. The output of the mixing `and amplifying device l8 is shown bythe wave form 2| and it will be noticed by inspecting this wave formthat the impulses occur at constant frequency but are of a varyingamplitude, the darker the subject the greater the amplitude. It willalso be noticed that the shape of each impulse wave corresponds to theshape of the wave form of the screen frequency `as shown at i9. theamount of each wave used depending upon the amount of output from therectifying device l5.

The output of the mixing and amplifying device !3 is then supplied to apush-pull keyer 24, one portion of the output being fed directly to thepush-pull keyer and the other portion of the output being suppliedthereto through a phase reverser 22 in order that the control electrodesof the push-pull keyer will be affected equally but in phase opposition.The separate halves of the push-pull keyer then supply wave forms suchas are shovm at 25 which comprise a series of impulses of constantfrequency and of constant amplitude but having a duration as determinedby the amplitude of the output of the mixing and amplifying device.These impulses are then directed to a keyed modulator 2S to which isalso supplied a line carrier frequency of the form shown at 2l which isgenerated by a line carrier oscillator 28. The output of the modulatoris then represented by the wave form shown at 29, which constitutes aseries of impulses occurring at a constant frequency, each impulseconsisting of a group of cycles at line carrier frequency and at aconstant amplitude, the number of cycles in each group varying inaccordance with the length of the impulses as supplied to the modulatorby the push-pull keyer. The output from the modulator may then betransmitted by radio or by line channel to a remote station for `ofwhich is connected a resistance 32.

subsequent reception by a receiver which transforms such impulses into areproduction of the original subject which was scanned.

Referring now to Figure 2, which shows a preferred form of the presentinvention, the signals as supplied by the scanner I l are impressed uponthe primary transformer 30, across the secondary This resistance is apotentiometer by means of which the intensity of the signals as suppliedto the amplifier tube 34 may be controlled. The control electrode of thetube 34 is maintained at an appropriate negative bias with respect tothe cathode by means of the resistance 36, across which is connected adecoupling condenser 38. The anode of the amplifier 34 is connected to asource of positive potential, the connection including the primary oftransformer 48. A choke member 4| is also included in this connection,in order that the alternating component of the anode current may beby-passed to the cathode of the tube 34 by the condenser 42.

The ends of the secondary of the transformer 4U are connected to the`anodes which form Athe dioderportion of the duo-diode triode tube 44.The midpoint along the secondary is connected to the cathode of the tube44 by way of a resistance 46. The diode portion of the tube 44 thenoperates as a full wave rectifier, the rectifier signal appearing acrossthe cathode resistor 46.

The rectifier signal is then fed through a suitable lter network whichcomprises a series of chokes 41 and condensers 48, the filterterminating in a resistance 50. The rectified and ltered signal is thenimpressed upon the control electrode of the triode section of the tube44, `upon which is also impressed a screen tone. The screen tone isapplied to the primary of the transformer 52, across the secondary ofwhich is connected a resistance which operates as a potentiometer inorder that the intensity of the screen tone vas applied to the controlelectrode of the triode section of the tube 44 may be controlled. Theanode of the triode section of the tube 44 is connected to a positivesource of potential through the resistance 51. Impulses of screen tonefrequency then appear in the anode circuit of the tube 44, the amplitudeof these signals depending upon the rectified current as supplied by thediode section of the tube 44.

The resistances 54, 55, 56, 51 and 58 form a voltage divider forsupplying the proper voltages to the various electrodes of the tubes ofthe circuit. The anode of the triode section of the tube 44 receives itspositive potential from a point between the resistances 56 and 51. Thecathode of this tube is connected to ground through resistances 55 andEll-in order that the anode current of the tube will cause potentialdrops to appear across these resistances. There-fore when current isdrawn through the anode circuit of the triode section of tube 44, avoltage will appear across resistances- 59 and 68 commensurate with theamount of the current drawn. v

The push-pull keyer 24 comprises a duplextriode tube 62 having a cathode64, control electrodes 65 and 66, .and anodes 61 and 68. The controlelectrode 65 is connected to the cathode of the duplex-diode triode 44,and the control electrode 66 of tube 62 is connected to a point betweenthe resistors 59 and 60 b-y way of a phase reverser 22. The phasereverser comprises a triode 16, the cathode of which is connected to apoint between resistors 54 and 55 and the control electrode of which isconnected to the point between yresistors 59 and 60. By so connectingthe tube 10, the control electrode is maintained at an, appropriatepotential with respect to the cathode thereof and the voltage dropswhich appear across the resistance as a result of anode current beingpassed by the triode section of the tube 441 are impressed upon thecontrol electrode of tube 10 to cause anode current to flow in the tube10 as the result of Ythese voltage variations. The anode of tube 10 isconnected through resistance 12 to a point between resistances 55 and 56in order that potentials which appear across the resistance 12 as aresult of the conductivity of tube 10, may be impressed upon the controlelectrode 66 of the tube 62.

The tubelll, because of its connection, therefore operates as a phasereversing tube and by its presence causes the grids and 86 of the tube62 to be operated in phase opposition in accordance with the currentpassed by the triode section of the tube 44.

The anodes 61 and 68 are connected tov a point between resistances 51and 58, the connections including resistances 1| and 12. Connected toeach of the anodes 61 and 68 are a pair of conductors 13 and 14`whichterminate at a pair of switch terminals for cooperation with a switcharm 15.

When either half of the push-pull keyer 24 is operating, a potentialdrop will be present across either the resistance 1I or 12, dependingupon which half of the keyer tube is drawing current at the particularinstant. This potential drop is applied to the grids of the push-pullkeyed modulator 26 through the switch 15. The keyed modulator tubecomprises a twin triode tube 11 which consists of a cathode, a` pair ofcontrol electrodes and a `pair of corresponding anodes. The carrierfrequency which is to be utilized in transmitting the-facsimile signalsis impressed upon' the primary of transformer 18, the ends of thesecondary of the transformer being connected to the two controlelectrodes of the modulator tube 11. The switch arm 15 is connected to amidpoint along the secondary of the transformer 18. The two anodes ofthe tube 11 are connected to the ends of the primary of transformer 80,the anodes being maintained at a positive potential through connection`to the midpoint of the primary. The secondary of the transformer 80 maybe connected to an appropriate transmitting or antenna system, or it maybe connected to a line channel for transmission of the modulatedsignals. The cathode of the tube 11 is connected to a point betweenresistances 51 and 58, the connection including a resistance 8l and acondenser 82.

The purpose of the switch 15 is to permit either positive or negativetransmission since it may be seen that if the switch 15 is in the lowerposition marked L, the transmission will be in a positive manner, thatis, absence of signal input will result in no output signal and presenceof signal input will permit an output to be present, whereas if theswitch 15 is placed in the upper position marked U the transmission willbe exactly the opposite, or in other words, negative, that is, thecondition in which no signal input results in maximum signal output.

In order to describe the operation of the system, it will be assumedthat a condition of no tion of the duo-diode triode 44, and therefore nopotential is present across either the resistance 46 or the resistance50, and accordingly no grid bias is applied to the control electrode ofthe triode section of tube 44. The triode section of this tube willtherefore draw plate current causing a voltage drop to appear acrossresistances 59 and 50. The presence of the voltage across theresistances 59 and 60 causes the control electrode 65 to be maintainedat a positive potential with respect to its cathode and thereforepermits this section of the push-pull keyer to conduct current. Thecontrol electrode of the phase reversing tube also receives a positivebias due to the potential drop across the resistance 66, which permitsanode current to flow in the tube lil and accordingly there appears apotential drop across the resistance vl2. As: the result of this, thecontrol electrode GE on the other half of the push-pull keyer will be ata negative potential with respect to its cathode by an amount sufficientto producing blocking thereof and accordingly there will be no currentdrawn by the anode 68 of the tube 62. The current from the anode 6l owsthrough the resistance ll and causes a potential to appear thereacrosswhich is impressed-assuming the switch l to be in the lower positionuponthe control electrodes of the modulator tube 'ISwith the result thatthis tube is biased to cut-olf and no current is passed therethrough.

As signals are applied to the amplifier tube 34 a potential drop willappear across the resistances 45 and 50 as a result of current beingpassed by the rectifier portion of the tubel 44, which, when applied tothe control electrode of the triode section of tube 44 will operate toreduce the amount of current passed by the tube and the length, as wellas the amplitude of the screen frequency impulses as they appear in theanode circuit of the tube 44 will depend upon the strength of theoriginal input signal. These impulses are passed on to the grid 65 ofthe pushpull keyer 62, causing anode current to flow through theresistor 1| and at each instant that current is permitted to flowthrough this anode circuit, a corresponding reduction in the anodecurrent of anode 68 is also present. The two halves of the push-pullkeyer then operate alternately depending upon the presence or absence ofcurrent in the triode section of tube 44 and the length of time that oneor the other half is conducting or non-conducting depends upon theamplitude of the impulses as supplied by the triode section of the tube44. Therefore when a signal is impressed upon the system, and when thetriode section of the tube 44 is nonconducting, there will be no currentflowing through the resistor 1I and the anode 6l. Because of the absenceof current in this resistor the potential of conductor 13 will be morepositive with the result that the control electrodes of the modulatortube 'l1 will be at such potential as to permit current to flow throughthe modulator tube, resulting in an output of modulated line carrierfrequency. As previously stated, the length of time that the modulatortube is permitted to operate depends upon the amplitude of the currentimpulses as passed by the triodel section of the tube 44, as may be seenby referring to the curve shown at 2| in Figure 1. As the signal inputis increased less and less of the screen tone becomes effective due to anegative bias being applied to the control electrode of the triodesection of tube 44 and the impulses transmitted as a result thereof willbe proportionately shorter. Finally, when the signal voltage hasincreased to such an extent as to cause a complete cut-off of the triodesection of the tube 44, then the opposite condition of a no signalcondition will exist, and a steady and continuous output from themodulator tube will result. It must be kept in mind that thisexplanation of the operation of the system would be exactly reversed sofar as the effect of the keyer tube on the modulator tube is concerned,if the switch 'l5 were placed in the upper or negative transmissionposition.

f it is desired to use negative transmission-in which case the absenceof a signal would result in maximum output from the modulator tubeit ismerely necessary to move the switch 'l5 from the lower position to theupper position and by so doing the modulator tube 'il is made to respondto voltage drops which appear across the resistance l2 rather than theresistance ll. These resistances, as may be seen from Figure 2, areconnected to opposite halves of the push-pull keyer tube 62.

Although the applicants system has been shown and described more or lessspecifically with respect to the type of tubes used, it is to beunderstood that other tubes might well be used to replace those shown inthe drawings. However, since all of the tubes shown are variable, t isdesirable that these tubes be used since the circuit may be materiallysimplified as a result thereof. By way of modification the tube 44 mightbe replaced by a half-wave or full- Wave rectifier and a triode orpentode by making appropriate changes in the system and both of thetubes 62 and 'II might be replaced by separate triodes rather thanresorting to the use of the combined tubes as shown by the figure.

Various other modifications and alterations may be made in the presentinvention Without departing from the spirit and scope thereof, and is isdesired that any and all such modifications be considered within thepurview of the present invention except as limited by the hereinafterappended claims.

I claim:

1.'A system for producing picture records comprising a discharge tubehaving a cathode, a pair of anodes, a control electrode and anadditional anode, means for impressing push-pull alternating signalvariations between said pair of anodes and said cathode forrectification thereby, means for applying a substantially constantalternating voltage of a predetermined frequency between said controlelectrode and said cathode, means for maintaining said additional .anodepositive with respect to said cathode, a load and filter circuitconnected in the cathode circuit of said tube across which impulses areproduced having a constant frequency corresponding to the saidpredetermined frequency and of an amplitude determined by the rectifiedsignal variations, a push-pull keyer comprising a pair of thermionicdischarge paths, means for rendering the discharge paths of saidpush-pull keyer alternately conducting and non-conducting for timeperiods determined by the amplitude of said impulses at the frequency ofoccurrence of said impulses, a carrier frequency modulating device, andmeans for controlling the operation of said modulating device inaccordance with the conducting or non-conducting condition of the one orthe other thermionic discharge path.

2. A system for producing picture records comprising a discharge tubehaving a cathode, a pair of anodes, a control electrode and anadditional anode, means including a cathode load resistance forimpressing push-pull signal variations between said pair of anodes andsaid cathode for rectification thereby, means for applying analternating potential of substantially constant frequency and amplitudeacross said control electrode and said cathode, means for maintainingsaid additional anode positive with respect to said cathode to produceat the cathode end of the cathode load resistance impulses of constantfrequency but of an amplitude determined by the rectified signalvariations, a push-pull keyer comprising a pair of thermionic dischargepaths, means responsive to the produced impulses for rendering the pathsof said push-'pull lreyer alternately conducting and non-conducting fortime periods determined by the amplitude of said impulses and at theconstant frequency of occurrence of said impulses, a carrier frequencymodulating device, and means for selectively connecting said modulatingdevice to the one or the other of said discharge paths whereby theoperation of said modulating device may be controlled by the conductiveor non-conductive condition oi the selected path.

3. A facsimile transmitting system comprising a discharge tube having acathode, a pair of ancdes, a control electrode and an additional anode,means for impressing pushpull signal variations between said pair ofanodes and said cathode for rectification, means for applying analternating voltage of a predetermined frequency between said controlel-ectrode and said cathode, means for maintaining said additional anodepositive with respect to said cathode, a cathode load and lter circuitconnected in the cathode circuit of said tube across which impulses ofsaid predetermined frequency and of an amplitude determined by therectiiied signal variations are produced, a pair of electronic dischargepaths, means including a phase inverter tube for controlling theconductivity of one of paths, means for controlling the phase invertertube and the conductivity of the other of said paths by the producedimpulses to produce, by the discharge paths, additional impulses of thesaid predetermined constant frequency and of constant amplitude but of aduration determined by the amplitudes of the first named impulses,

a carrier frequency modulating device, andy means for controlling theoperation of said modulating device in accordance with the saidadditional impulses as supplied by the one or the other of saiddischarge paths.

4. A facsimile transmitting system comprising a discharge tube having acathode, an anode, a control electrode and an additional anode, meansfor impressing alternating signal variations across said cathode andanode for rectification thereby, means for applying an alternatingvoltage of a predetermined substantially constant frequency across' saidcontrol electrode and said cathode, means for maintaining the saidadditional anode positive with respect to said cath ode, a loadimpedance in the cathode circuit of said tube across which a first seresof impulses of the said predetermined constant frequency but of varyingamplitude and time duration, in accordance with the rectified signalvariations, is produced, a pair of electronic discharge paths, meansincluding a phase inverter device for differentially controlling theoperation of said paths to produce a second and a third series ofimpulses of constant frequency and amplitude but of a durationdetermined hy the amplitude of rst series of impulses, a series ofcarrier frequency oscillations, and means for modulating saidoscillations in accordance with said second or said third series ofimpulses.

5. A system for producing picture records comprising a discharge tubehaving a cathode, a pair of anodes, a control electrode and anadditional anode, means for impressing alternating signal variationsbetween said pair of anodes and said cathode for rectication thereby,means for applying an alternating voltage of a predeterminedsubstantially frequency and of a substantially constant amplitudebetween said control electrode and said cathode, means for maintainingsaid additional anode positive with respect to said cathode, a loadimpedance connected in the cathode circuit of said tube across whichimpulses of constant frequency and of an amplitude determined cy therectified signal variations are produced, an electronic keying devicecomprising a cathode, a control electrode and an anode, means forapplying said impulses to the control electrode of said device toproduce impulses of said constant frequency and of a substantially linedamplitude but of variable duration in accordance with said signalvariations, means including a load circuit for maintaining the anode ofsaid keying device positive with respect to its associated cathode, anda carrier frequency modulating device responsive to the voltage dropacross said load circuit.

6. A facsimile transmitting system comprising a discharge tube having acathode, a pair of anodes, a control electrode and an additional anode,means for impressing alternating push-pull signal variations across saidancdes and said cathode for rectification thereby, means for applying analternating voltage of a predetermined frequency across said controlelectrode and said cathode, means for maintaining said anode positivewith respect to said cathode, a load impedance and a lter network in thecathode circuit of said tube across which a first series of impulses ofconstant frequency but of varying amplitude and time duration inaccordance with the rectified signal variations are produced, anelectronic discharge device, means for controlling the operation of saiddevice to produce a second series of impulses of constant frequency andamplitude but of a time duration determined by said first series ofimpulses, a source of carrier frequency oscillations, and means formodulating said oscillations in accordance with said second series ofimpulses.

JAMES N. 'WHILTAKER.

